This invention relates to a method and apparatus for extending the life of a control or drive spring in a wrap spring clutch.
Drive springs are used in overrunning clutches, on-off clutches, and single revolution clutches, for example. In general, the control or drive spring, which is also called a "wrap spring", is used to provide the driving coupling between an input hub and an output hub or shaft of the associated clutch. When the clutch is actuated, the drive spring is tightened around the input and output hubs forming the driving coupling or connection therebetween.
The single revolution clutches mentioned, or clutches which provide two or more stops per revolution of the input member, are provided generally with a drive spring and a brake spring. The drive and brake springs each have a tang which is bent outwardly in a radial direction. The inner ends of the drive and brake springs are connected to the output member of the clutch, and the tangs of the drive and brake springs are mounted in separate control slots in a common control collar which surrounds the output member and the two springs. The drive and brake springs are wound in opposite directions so that when the clutch is actuated by releasing the control collar, the control spring "tightens down" on a rotating input sleeve and the output member while the brake spring "opens up" or loosens, permitting the rotary motion of the input sleeve to be transferred to the output member.
When the clutches mentioned in the previous paragraph were run at the upper end of the associated input speed range, they failed, persistently, at about one million cycles of operation. The failure was due, generally, to the tang of the drive spring breaking. Failure of the clutch at about one million cycles of operation was too short a life for a particular application in which the clutch was used.
Numerous attempts by clutch vendors and others to change the wire size of the drive spring, the wire material, and the wire geometry, for example, failed to provide a solution which prolonged or extended the life of the drive spring and the associated clutch.